What you experienced was the second common failure mode of lead acid batteries.  This is called sulfation, and is quite common in equipment that’s stored during the off-season—boats, golf cars, RVs, lawn tractors are just some of the pervasive examples which experience this type of battery failure.  When a lead acid battery is kept in a discharged state for an extended period of time, lead sulfate compounds formed in the internal chemical reaction precipitate out of the electrolytic solution and coat the grids’ surface.  As these sulfates are non-conductive, they block the electron path that is necessary for recharging and the battery becomes non-serviceable.  In normal use, as the battery is recharged, these compounds are converted back into their basic chemical components.  During long periods of inactivity, however, they become battery killers.